1. Field of the Invention
The present invention relates to a horizontal S-shape correction circuit suitable for a display unit using an image receiving tube, and more specifically to a technique for reducing M-shape distortion such that the horizontal linearity is reduced at the central portion of an image during a wide angle deflection, and further, when a plurality of sorts of horizontal deflection periods are USED, for correcting the other image distortions such as S-shape distortion of an optimum horizontal linearity, inner pin cushion distortion, etc. for each horizontal deflection period.
Further, the present invention relates to a horizontal S-shape correction circuit suitably used for a display unit so designed as to be operative at various horizontal deflection frequencies. The horizontal S-shape correction circuit according to the present invention can automatically set an optimum horizontal deflection frequency so that the S-shape distortion correction of the horizontal linearity can be optimized for each of a plurality of the horizontal deflection frequencies.
2. Description of the Related Art
Conventionally, when an image receiving tube having a relatively wide flat image receiving surface and a wide angle deflection is used, the horizontal linearity distortion has a tendency of M-shape distortion such that the distortion is reduced at the central portion of the image receiving surface, as shown by solid line in FIG. 1. This tendency is prominent in particular in the wide image receiving tube with an aspect ratio (vertical and horizontal ratio) of 9:16, so that there exists a need of improving this problem.
FIG. 2 shows an example of the improved circuits, which is composed of a horizontal deflecting coil 1, an S-shape correcting capacitor 2, and a secondary resonance circuit 3 for correcting the M-shape distortion. In general, however, since the secondary resonance circuit 3 is not used, an electron beam of the image receiving tube is deflected right and left, when a saw tooth horizontal deflection current flows through a series circuit composed of the horizontal deflecting coil 1 and the S-shape correcting capacitor 2.
In FIG. 2, the linearity thereof can be corrected by changing the beam extension toward the right and left relative to the central portion of the image receiving surface, that is, by adjusting the capacitance value of the S-shape correcting capacitor 2. In this case, however, as shown by dashed lines and dot-dashed lines in FIG. 1, it is impossible to remove the central shrinkage portion by adjusting only the capacitance of the correcting capacitor 2. This is because there exists a relatively big difference between the optimum current waveform and the current waveform adjustable by depending upon the operation of only the S-shape correcting capacitor 2.
To overcome this problem, the secondary resonance circuit 3 is added to correct the M-shape distortion. The secondary resonance circuit 3 is composed of a coupling capacitor 4, a resonance capacitor 5, and a resonance coil 6, which can resonate at a frequency twice higher than that of the horizontal deflection frequency. In this case, since the voltage across the S-shape correcting capacitor 2 changes from a parabolic waveform (obtained when the secondary resonance circuit is not connected) as shown by a solid line in FIG. 3 to a waveform (obtained when the secondary resonance waveform is superposed upon the parabolic waveform) as shown by a dashed line in FIG. 3, with the result that the M-shape distortion as shown by the solid line shown in FIG. 1 can be corrected.
Further, when the display unit is used as a display terminal of a computer, various horizontal deflection frequencies are usually needed according to the set resolution of the computer signals.
FIG. 4 shows a related art example of correcting the S-shape distortion used for the display terminal of a computer. In FIG. 4, an auxiliary capacitor group composed of capacitors 7-1, 7-2 and 7-3 is added to the S-shape correcting capacitor 7. Further, each end of each capacitor of the auxiliary capacitor group is grounded through each of three electronic switches 8-1, 8-2 and 8-3, respectively. Further, these electronic switches are controllably turned on or off, respectively in response to three control signals applied externally.
In FIG. 4, when the horizontal deflection frequency is the highest, all the electronic switches 8-1 to 8-3 are turned off, and the capacitance of the main S-shape correcting capacitor 7 is so decided that an appropriate correction can be obtained by only the S-shape correcting capacitor 7. Further, when the horizontal deflection frequency is reduced, the electronic switches 8-1, 8-2 and 8-3 are changed from turn-off to turn-on in sequence, to connect each auxiliary capacitor 7-1, 7-2 and 7-3 in parallel to the main S-shape correcting capacitor 7 in sequence. Here, the total capacitance value of all the S-shape correcting capacitors is set to a value suitable for the horizontal frequency at that moment.
In this method, however, since the optimum frequency decided by the total capacitance is limited by the number of the electronic switches, the frequency is usually corrected only approximately, so that the correction quality inevitably deteriorates. On the other hand, although the correction quality can be improved by increasing the number of combinations of both the switches and the S-shape correcting capacitors, in this case, however, the circuit scale inevitably increases.
Further, in FIG. 4, it is possible to add the aforementioned secondary resonance circuit 3 for the M-shape correction. In this case, however, since the values of the coil and capacitor of the secondary resonance circuit 3 must be switched whenever the horizontal frequency changes, the circuit scale is further increased.
On the other hand, in addition to the above-mentioned horizontal linearity distortion, the image receiving tube having a wide angle deflection and a flat image receiving surface involves a problem related to inner pin cushion distortion as shown in FIG. 5. This distortion implies such a phenomenon that even if the side pin cushion is corrected so that the vertical lines are straight on both right and left ends on the screen, since the pin cushion remains at the intermediate portion thereof, the vertical lines are curved inward at the central portion of the screen surface.
This distortion indicates that the horizontal linearity of both sides extends largely in the scanning lines near the central portion of the screen, as compared with the upper and lower portions of the screen. Therefore, it is necessary to apply a strong S-shape correction at the central portion, as compared with that at the upper and lower ends thereof. However, it has been so far very difficult to adjust the horizontal S-shape correction rate according to the vertical position.
Further, when the horizontal amplitude is changed largely (e.g., under scanning or over scanning of an image), even if the optimum S-shape correction can be made for the normal horizontal amplitude, there exists a problem in that the S-shape correction is too sufficient in the case of the under scanning but insufficient in the case of the over scanning. Of course, although it may be considered to switch the values of the S-shape correcting capacitors according to the scanning rate by use of the switch group as shown in FIG. 4, in this case, however, the circuit is further complicated to that extent.
As described above, in the prior art technique, when a further accurate S-shape correction is required or when an appropriate S-shape correction is needed for each of various horizontal deflection frequencies, there exists a problem in that the circuit scale is huge for the horizontal S-shape correction including the M-shape correction. In addition, it is impossible to obtain an optimum correction all over the corresponding frequency ranges. To overcome this problem, it is necessary to develop new S-shape correcting means for changing the correction rate continuously, without depending upon the switching operation as is conventional.
Further, with respect to the inner pin cushion distortion correction, it is necessary to use S-shape correcting means for changing the correction rate continuously according to the vertical position. Therefore, there exists a need of realizing a simple and loss-less circuit for executing the above-mentioned continuous and variable S-shape correction.
Further, as is well known, when an image receiving tube having a relatively flat image receiving surface and a wide angle deflection is used, it is possible to obtain a correct horizontal linearity only when the S-shape corrected current waveform is used (i.e., the current slope is reduced on the right and left side as compared with the central scanning portion), without simply passing only the saw tooth waveform through the horizontal deflecting coil. For this purpose, in the prior art circuit, an S-shape correcting capacitor is inserted in series to the horizontal deflecting coil, so that the above-mentioned purpose can be obtained by the resonance operation.
However, in the case where the horizontal deflection frequency changes according to the set resolution as in the computer display terminal, it is impossible to cope with this problem by use of only a single value of the S-shape correcting capacitor, so that it has been necessary to set the value of the S-shape correcting capacitor to an optimum value according to the horizontal deflection frequency. Therefore, the value of the S-shape correcting capacitor has been so far switched by a method as shown in FIG. 6.
FIG. 6 shows a related art horizontal deflection output circuit for switching the S-shape correction capacitance value, in which a deflecting coil current Iy is passed through a horizontal deflecting coil 1 according to a driving pulse Vd synchronized with the horizontal deflection frequency of an input signal (applied from a front stage (not shown)). Here, the reference numeral 7 denotes an S-shape correcting capacitor. When the normal single horizontal deflection frequency is used, the following auxiliary S-shape capacitor group 7-1, 7-2 and 7-3 are all not necessary, so that only the S-shape correcting capacitor 7 is used alone.
On the other hand, when the various horizontal deflection frequencies are used, a plurality of auxiliary S-shape correcting capacitors 7-1, 7-2 and 7-3 (auxiliary S-shape correction capacitor group) connected in parallel to the main S-shape correcting capacitor 3 are added. Here, each end of each auxiliary S-shape correcting capacitor is grounded via each of electronic switches 8-1, 8-2 and 8-3. Further, these electronic switches are controllably turned on or off on the basis of a plurality of external control signals Vsw. In FIG. 6, when the horizontal deflection frequency is the highest, all the electronic switches 8-1 to 8-3 are turned off, and the capacitance of the main S-shape correcting capacitor 7 is so decided that an appropriate correction can be obtained by use of only the capacitor 7. Further, when the horizontal deflection frequency is reduced, the electronic switches 8-1, 8-2 and 8-3 are changed from turn-off to turn-on in sequence, to connect each of the three auxiliary capacitors 7-1, 7-2 and 7-3 in parallel to the main S-shape correcting capacitor 7 in sequence. Here, the total capacitance value of all the S-shape correcting capacitors is set to a value suitable for the horizontal frequency at that moment.
Here, these electronic switches are controllably turned on or off on the basis of the external control signals Vsw. These control signals Vsw are so set that the most appropriate electronic switch circuits can be turned on according to the sort of the input signal (according to the sort of the horizontal deflection frequency of the input signal applied to the display unit).
In this prior art method shown in FIG. 6, however, since the total capacitance value of the S-shape correcting capacitors can be only changed stepwise, the optimum horizontal deflection frequency is inevitably limited to the number of the electronic switches. That is, since the frequency is usually corrected approximately (except when the optimum horizontal deflection frequency is obtained), the correction quality inevitably deteriorates. On the other hand, although the correction quality can be improved by increasing the number of combinations of both the switches and the S-shape correcting capacitors, in this case, however, the circuit scale inevitably increases.
Therefore, when a technique is realized such that the S-shape correction rate can be changed continuously and further that the optimum S-shape correction rate can be decided automatically according to the horizontal deflection frequency, there exists a large advantage from the standpoints of the performance improvement and the simplification of the circuit scale.